Schottky diodes are important passive components in, for example CMOS ICs that perform radio frequency (RF) and mixed signal (MS) functions. CMOS Schottky diodes include two terminals, an anode and a cathode, that are formed on a surface of the CMOS integrated circuit substrate (e.g., monocrystalline silicon), and also include isolation structure positioned between the anode and cathode. The Schottky diode consists a Schottky barrier, which is a metallic region, in direct contact with a relatively lightly doped semiconducting region, a method providing an ohmic contact to that lightly doped semiconducting region, which will be called a backside contact, and the structures necessary to define and isolate the two different contact regions. According to the choice of substrate doping and the metallic material, the metallic region may be either a cathode or an anode. When the metallic material is in contact with a P-type region, it takes the role of cathode. On the other hand, if the metallic material is in contact with N-type silicon, it takes the role of an anode. In either case, the Schottky diode is completed with an ohmic contact to the underlying semiconductor region. When the Schottky diode is biased so that the anode is positive with respect to the cathode, and when a sufficient bias voltage exists between the Schottky barrier and the Ohmic contact, a relatively high current is produced that passes through the intervening substrate. When the anode is biased negatively with respect to the cathode, a much reduced, reverse current flows. Like all diodes, the Schottky diode is subject to breakdown if excessive reverse voltage is applied. The magnitudes of the forward and reverse currents are determined first by the choice of N-type or P-type semiconducting material, second by the choice of metallic material, third by the doping density of the semiconducting material, and finally by the details of the device geometry.
Due to the general trend toward RF and MS CMOS ICs that function at ever-lower operating voltages, there is a need for passive components, such as Schottky diodes, that exhibit a sufficiently low turn on voltage and low series resistance. In addition, there is a need for passive components, such as Schottky diodes, that are fabricated to minimize parasitic resistance and capacitance, which would impair the operation of the circuits they support. These Schottky diode operating characteristics can be “tuned” to a desired level through the selection of either N-type or P-type doping for the semiconducting region, the selection of metallic material used to form the Schottky barrier and ohmic contact, the doping levels of the diode well, and the distance between the anode and cathode (i.e., the width of the isolation structure).
Schottky diodes were originally used in bipolar ICs, where metal junctions were formed on regions of the underlying silicon substrate doped with an N-type dopant. The turn-on voltage of these Schottky diodes was determined by characteristics of the selected metal, and by the doping density of the underlying silicon regions. Schottky diodes are utilized in RF and MS ICs, for example, to produce voltage multipliers, RF mixers, and efficient high speed rectifiers. However, the operation of Schottky diodes that are made using conventional methods suffers from parasitic capacitance and leakage to the underlying substrate. Such a leakage path can bypass the diode, degrading the diode's rectifying or nonlinear behavior. Further, when product or design consideration dictate that the Schottky barrier be formed with respect to a region, P-type or N-type, which has the same doping character as the substrate, one terminal of the Schottky diode will be directly shorted to the substrate, unless measures are taken to isolate that device.
What is needed is a method for fabricating Schottky diodes for low voltage RF and MS circuits whereby the parasitic capacitance and conduction to the underlying substrate is substantially minimized. What is needed is a Schottky diodes in which series resistance is minimized in order to facilitate low voltage operation. What is also needed is a method for producing such Schottky diodes that can be incorporated into a standard CMOS processing flow and in a manner that reduces the number of additional processing steps, thus maintaining the lowest production cost possible for such a diode and the circuits incorporating the diode.